1. Field of the Invention
The present invention relates to electroless nickel plating solutions and a method for their regeneration. More particularly, the present invention relates to electroless nickel plating solutions and regeneration methods for indefinite use of the solutions without discharge of hazardous waste.
2. Description of the Prior Art
Electroless nickel (EN) plating is known and is useful for generating corrosion and wear resistant coatings on metal or plastic parts, both on open surfaces or inside cavities. Common applications are for electronics, computers, valves, aircraft parts, copier and typewriter parts and printing press rolls.
In most electroless nickel plating baths the nickel ions are catalytically reduced by sodium hypophosphite, which is oxidized to phosphite in the redox reaction. Due to side reactions, three moles of hypophosphite are required to reduce one mole of nickel. The reactions proceed in the presence of a catalyst as follows: ##STR1##
Thus, for each gram of plated nickel-phosphorus alloy about 5 g of sodium orthophosphite is generated in the plating bath. If air is used to agitate the bath, an additional 5 to 10 per cent of the hypophosphite is oxidized.
During the plating process the nickel and hypophosphite are continuously depleted and must be replenished in order to maintain the chemical balance of the bath. Plating quality and efficiency decrease as the phosphite level increases in the solution and it becomes necessary to discard the plating bath, typically after about 25 grams of nickel gave been plated per liter of EN bath. Treatment of the spent solution is desirable, both to improve the economics of the process and to avoid nickel-containing hazardous waste disposal.
It has been proposed to remove phosphites from the spent EN plating bath either through precipitation techniques or treatment in a weak-base anionic ion exchange resin column to remove undesired phosphites, making the bath usable for further plating operations. (See Konrad Parker, Plating and Surface Finishing, vol. 67, p. 48(March, 1980)) Another ion exchange process has been proposed using strong-base ion-exchange resin in the hypophosphite form (pretreated with sodium hypophosphite) to exchange anions including phosphite with bound hypophosphite. (See F. Levy et al., Plating and Surface Finishing, vol. 74, p. 60(September 1987))
A relatively complex procedure combining ion exchange and precipitation has been proposed which reaches the desirable goal where all chemicals entering the plating bath are removed after their purpose has been served. (See R. W. Anderson et al, Plating and Surface Finishing, vol. 77, p. 18(March 1992) and U.S. Pat. No. 5,112,392 (May, 1992)).
The present invention overcomes the deficiencies in the above processes by providing a simplified procedure for treating spent plating bath solution which allows its perpetual use and avoids all hazardous waste discharge. This is accomplished by providing a novel EN plating bath composition compatible with the regeneration process. The novel EN plating bath of the present invention is comparable with existing commercial baths in terms of quality and ease of production, but is also suitable for a regeneration process which enables its perpetual use while avoiding hazardous waste disposal. A novel concentrated starter and replenishment solution is provided for ease in forming and operating the EN bath of the present invention.
Existing commercial EN baths typically reach end-of-life after the original nickel content has been replaced four times through replenishment. Since normal operation acidifies the bath, neutralization with ammonium hydroxide is common practice. This leads to buildup of neutral salts and causes a decline in the plating performance and, eventually, necessitates disposal of the bath. Commercial baths are not suitable for the inventive regeneration scheme because (1) the presence of ammonium salts interferes with the precipitation of calcium compounds, precluding consistent, high-quality nickel plates, and (2) conventional EN baths depend on frequent or continuous replenishment for stable performance, which are incompatible with the regeneration scheme. For optimal performance in a plating and regeneration cycle, an EN bath must show minimal drift in performance over extended usage. This can be accomplished by stabilizing the solution pH. Commercial EN baths may contain lactic acid or other organic acids with a low buffer capacity for the usual operating pH of 5. These acids are also undesirable if calcium compounds can be formed that precipitate during the regeneration process and lead to an undesirable change in bath composition. In contrast, the novel bath composition of the present invention contains acetic acid/acetate as a buffer, which has a ten times higher buffer capacity than lactic acid in the relevant pH range, and forms a highly soluble calcium salt which will not precipitate during regeneration.
The fundamental shortcoming of the -392 process is that it is too complicated, involves too many processing steps, and is not completely effective. The chemical principles that govern the functions of electroless nickel plating are as follows: (1) Electroless nickel baths operate either in acidic or in alkaline medium. Only the acidic type is conducive to rejuvenation with the methods considered by the -392 patent. These baths contain hypophosphite and nickel ions as primary ingredients. (2) Control of the pH is crucial for constant quality of the deposit and extended life of the bath. During use, the bath pH drifts to lower values, which causes an increase in the rate of phosphorus codeposition. When pH corrections are made by adding sodium hydroxide, there is a chance that nickel will precipitate, which, in turn, may initiate spontaneous plating and result in the destruction of the bath. This possibility can be minimized by adding ammonium hydroxide, instead of sodium hydroxide, except that the bath becomes unsuitable for regeneration as a result. Another path for circumventing the decomposition during pH adjustments is to include a complexing agent into the bath make-up, but again, the bath becomes less suitable for the regeneration procedure of either the -392 patent or the instant invention. The preferred method for maintaining the pH within reasonable bounds is to include a pH buffer in the bath. The -392 patent chose lactic acid/lactate as buffer system. Unfortunately, this buffer is best suited for a pH of 3.1, and is completely ineffective for the desired level of 5. The process of the -392 patent risks losing the bath through precipitation and spontaneous plating along with the inconvenience of making additions of sodium hydroxide.
The instant invention employs acetic acid/acetate buffer, which is inexpensive, yet excellent for the desired pH. Its buffer maximum is at pH 4.8. Other buffers for this pH regime exist, such as propionic, butanoic, pentanoic acid and others but, for an electroless nickel bath these alternatives must have no tendency to form a complex or an insoluble compound with nickel ions. This, along with cost, severely limits the choice of alternative buffers. Nickel lactate, incidentally, is sparingly soluble and precipitates at pH 5.5 to 6. Employing a buffer that accurately matches the desired pH range has the advantage of sodium hydroxide additions being superfluous. The present invention exploits this feature by avoiding additions of sodium hydroxide additions in the first instance, and therefore avoids the need for removing sodium ions during the recovery process.
The operation of the EN plating system of the -392 patent requires the removal of sodium ions along with sulfate and phosphite ions. Only phosphite needs to be removed in the process of the present invention. When attempting to remove phosphite from the nickel solution by precipitation, one faces the problem of losing some or most of the nickel by coprecipitation. Researchers have tried to design selective precipitation conditions, in which nickel does not participate (See Parker, above), however, the precipitant consistently spoils the quality of subsequent nickel plates. The desirable approach is to first remove the nickel from the solution, followed by precipitating the phosphite, and finally, recombining nickel with the filtered solution, the method accomplished in the present invention by removal of nickel, only, by reaction with a chelating ion exchange resin. The effluent from this process is nickel-free, hence nonhazardous and suitable for disposal in many publicly owned water treatment works. This effluent, however, is also suitable for selective removal of phosphite ions by precipitation with calcium hydroxide. If carried out with reasonable care, the pH will rise to about 13, no calcium will be passed into the filtrate, and the performance of the reconstituted nickel bath will not be degraded by the presence of calcium ions. The nickel ions are removed from the ion exchange resin by elution with hypophosphorous acid, the resulting eluate adjusted for nickel content and combined with the above filtrate, and the combined solution adjusted by evaporation to form the concentrated starter and replenishment solution of the present invention.